Bearing and lubricant film test method and apparatus

ABSTRACT

The bearing and elastohydrodynamic lubrication film test method and apparatus provides measurements related to acoustic resonance frequencies of the ball - raceway - lubricant system of a ball bearing, which frequencies are found to be related to bearing ball, race, and lubricant behavior, the measurements being made while the bearing is actually operating in its normal environment such as for example in a production gyroscopic instrument assembly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to test apparatus and to methods for the investigation of bearings or of their lubricants and more particularly concerns versatile test equipment and methods for the use thereof permitting testing of ball bearings and lubricants therefor under substantially the actual conditions in which they are expected to be used in commercial products such as gyroscopes. Furthermore, the apparatus and method are adaptable for performing fundamental research upon ball bearing configurations, including ball cages, or lubrication techniques and the lubricants themselves.

2. Description of the Prior Art

In the prior art, it has long been recognized that the proper generation and maintenance of the lubricant film between the bearing balls and races of an operating ball bearing assembly are critical with respect to bearing life and maintainability. This criticality is particularly present with bearings operating at high speeds or at relatively high temperatures. As a consequence, there have been many attempts in the prior art to measure elastohydrodynamic lubricant film thickness in actual bearings or in apparatus in which the bearing is simulated. Some of these measurement schemes have involved the electrical measurement of resistance, of dielectric breakdown voltage, or of the effective capacitance of the lubricant film under operating conditions. In addition, x-ray diffraction techniques and optical interferometric methods have been applied with certain degrees of success.

Because of their nature, most of the prior art test arrangements have involved specially prepared and instrumented bearing assemblies which usually merely simulate the actual bearing interfaces and thus do not give a reliably true picture of the behavior of an actual bearing lubricated with a particular lubricant in a situation which closely resembles the behavior of the film and bearing elements in a normal operating circumstance. On such example of a test device involving bearing simulation is taught in the T. B. Howe U.S. Pat. 3,178,928, issued Apr. 20, 1965, and assigned to Sperry Rand Corporation. In the Howe apparatus, important characteristics of ball bearings or elements thereof may be investigated by a simulation method so that valuable data may be obtained through measurements of torque and electrical resistance, the measurements providing an indication of operating efficiency. Since the apparatus is a simulation apparatus, interchangeability of simulated bearing components is permitted to some extent. The device provides output data respective to the torque transmitted through the ball elements as well as the electrical resistance to currents passing through the lubricating film associated with those ball elements. Changes in efficiency of the simulated bearing are generally indicated by rising or erratic torque or electrical resistance indications that can be used to predict failure well in advance of the failure event.

However, like many of the prior art test concepts, studies using the Howe apparatus are performed on simulated bearing arrangements and the concept does not readily permit the direct testing of commercially produced bearings or the testing of lubricants directly in such commercial bearings. In addition, it is not possible to test lubricants or bearings on which rotor assemblies, such as gyroscope rotors, are mounted in the form of production assemblies. Such tests are clearly desired, since some of the most influential and erratic sources of undesirable loading in ball bearings are those resulting during assembly of a bearing into the instrument in which it is to be used, such as in a gyroscope rotor or precision motor assembly. In general, the nature of such prior test devices precludes studies of bearings in actual operating conditions; they are basically research tools, while the present invention may be used either as a research tool or as a quality control monitor in design and production.

The reliability and life of a ball bearing is a function of its lubricating film thickness with respect to minute irregularities in the surface finish of the bearing balls and races. The film thickness of consequence is that of the plateau at the central contact locus of the lubricating film. The trailing edge of this separating film as a bearing ball passes a given location on a race is reduced in thickness by the hydraulic, elastic, hydrodynamic, and other forces involved. It has been determined experimentally that bearing life is most closely associated with the lubrication film thickness in the central plateau region and is not significantly related to that at the exit region restriction which is normally measured by electrical resistance measurements. Thus, electrical resistance measurements, while useful for many purposes, are often not as accurate an indicator of bearing life as is desired. Furthermore, it is not possible to search a bearing race for local imperfections, such as for areas in which a sufficient lubrication film does not exist due to an oleophobic surface, misalignments, or local raceway surface defects.

It is recognized that ultrasonic energy has been employed in the past for various testing purposes, such as, for example, the detection of hidden flaws deep within solid structures. In such conventional ultrasound test equipment, for example, a pulse of energy is directed through a medium toward a defect or other interface and the time of arrival of the reflected wave at a receiver is measured in order to afford a measurement of the depth of the flaw or other interface. However, the technique lends itself to use only in situations in which the flaw or interface to be observed is buried relatively deeply within the medium. Thus, the direct measurement of films as thin as bearing lubrication films would not prove to be practical using this conventional technique. Accordingly, it is believed that the prior art does not include satisfactory ultrasonic methods for the investigation of rotating or stationary ball bearing assemblies and of the lubricant films usually employed within them.

SUMMARY OF THE INVENTION

The bearing and elastohydrodynamic lubrication film test apparatus and method of the present invention provide measurements generally related to the individual acoustic resonant frequencies of the bearing balls as modified by the presence of the raceways of the bearing and the lubricant, which measurements are related to bearing and lubricant behavior factors that largely determine bearing life. The measurements are made while the bearing is actually operating in the normal manner in its normal environment, such as within a production instrument assembly. The measurements depend upon the fact that the bearing balls are each preloaded by the combination of respective ball, raceway, and lubricant spring effects at each instantaneous position of the movable elements of the bearing. Because the masses of each of the bearing balls of a given bearing assembly may normally be assumed to be equal, the ball-race-lubricant system resonant frequency associated with each ball may properly be related to bearing and lubricant characteristics.

In general, the invention consists in applying at a predetermined angular position, or a plurality of such positions, on the fixed bearing race of a normally operating bearing pulses of high frequency ultrasonic energy to excite the race-ball-lubricant system to resonance, and then detecting the resulting frequency characteristics of the response thereto, this response exhibiting characteristics relatable to various geometric or lubricant defects within the bearing system.

The invention overcomes the aforementioned problems of the prior art and is adapted to the measurement of a wide range of bearing geometries and lubricant film characteristics. For example, a complete search of a bearing race may be made to reveal geometric irregularities of the race, or partial or complete non-wetting raceway surface conditions and the like, or each ball may be examined as it passes a predetermined point on the fixed race for detecting ball geometries, lubricant effects, or the like. Raceway alignment, stationary or dynamic, may be determined. The thickness of the lubricant film throughout the bearing may be determined, as well as its integrity with respect to factors such as speed, load, and temperature. The apparatus may also be used in the study of the effects of lubricant additives or of the consequences of the loss of components of high volatility normally present in a given lubricant sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the novel bearing test apparatus showing cooperating electrical devices and the electrical connections thereto and is illustrative of one embodiment of the novel method.

FIG. 2 is an elevation view of the part of the apparatus of FIG. 1 for support of a device to be tested.

FIG. 3 is a plan view, partially in cross section, of the ultrasonic driver 3 of FIG. 1.

FIG. 4 is a plan view of isolated elements of the transducer of FIG. 3 useful in explaining operation of the invention.

FIG. 5 is an enlarged plan view of a part of FIG. 3.

FIGS. 6 and 7 are graphs useful in explaining representative capabilities of the invention.

FIG. 8 is a cross section plan view of a typical gyroscope rotor assembly under test according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles of the invention may be illustrated by way of example in application in a system such as that of FIG. 1 capable of examining the behavior of bearings and lubricants operating under the actual conditions in which they are to be used in representative commercial equipment. For example, tests may be made according to one aspect of the invention on bearings actually supporting operating gyroscopic rotors. As in FIG. 1, the rotor assembly 1 to be tested includes within its casing a complete gyroscopic rotor 6 supported on bearings indicated at 7 and 7a. The rotor assembly 1 may be exactly the assembly which would ordinarily be mounted in gyroscopic gimbals or a motor or other device. In the following description, the test method is carried out using a specially designed test cradle for receiving the rotor-bearing assembly. However, it will be understood, that in its broader aspects, the test method may be equally well applied to fully assembled rotary apparatus or to partially assembled apparatus, and that it may even be carried out with a rotor-bearing assembly hand held with the ultrasonic driver also hand positioned for the test. In the illustrated test apparatus, the rotor-bearing assembly is not in a gimbal system, but is supported by a flexible support member or cradle 2, the outer races of the bearings 7 and 7a or other part of the assembly to be tested being held in clamped relation by the mount frame 14. The mounting surface 32 of FIG. 2 may be arranged to attach directly, as will be explained, to a surface of the cage or hub of the rotor assembly, such as to surface 124 of FIG. 8, for example. The mount 2 has sufficient flexibility at right angles to the spin axis 1a of the gyroscope rotor 6 that certain high frequency vibrations present will be translated from the rotor assembly 1 to the flexible support 2. It will be seen that support 2 acts generally as a high pass mechanical filter.

Thus, the rotor assembly 1 is mounted in a cradle or flexible support 2 which has high mechanical stiffness along all axes except that of the vibration pick-off 5, which pick-off is sensitive to vibration displacements in the direction normal to the rotor spin axis 1a, being rigidly supported with respect to base 8. The support member 2 is preferably tuned so as to have a vibration response along the desired axis to signals above a predetermined high frequency of the order of 10 Hz. to 10 kHz., for example, depending upon the rotor mass, the bearing-ball complement, the spin frequency, and the like. Vibration pick-off 5, mounted rigidly with respect to base 8 so as to sense accelerations produced by dynamic forces within the bearing normal to the spin axis 1a, supplies its output to a preamplifier 11 and then to an adjustable notch filter 12. Notch filter 12 is a conventional circuit for passing signals in a narrowly restricted frequency range and is made adjustable by adjustment 12a since the signal sensed by pick-off 5 will vary in frequency in dependence upon the nature of bearings 7, 7a and the speed of rotation of rotor 6. Any signal passing notch filter 12 will be used after amplification by power amplifier 13 in apparatus and for purposes yet to be described. The principal frequency of the signal detected by pick-off 5 is related to the rate at which rotor 6 is operated. However, for purposes of the present invention, other secondary frequencies are also significant. One frequency is related to the passage of the individual bearing balls and, therefore, is a function of the number of bearing balls within the bearing. This frequency is found to lie between 0.3n and 0.35n times the frequency of rotation of the rotor 6, where n is the number of balls in the bearing under test and the rotor is mounted on the bearing inner race. This frequency is referred to herein as the ball pass frequency. Another frequency is the ball set rotation frequency, which may also be identified as the bearing cage rotation frequency.

The signal supplied by power amplifier 13 may be employed for several purposes; for example, it is used to trigger the light 16 of a convention stoboscopic light generator 15. In this manner, with switch 33 closed and operating at a predetermined ball set or cage rotation frequency, the operator may visually examine the location of a particular ball of the bearing 7 complement so that he may properly locate the probe 10 of ultrasonic pulse generator 3 as the test proceeds, or he may, by selecting the ball pass frequency by adjustment of notch filter 12, observe the position of the balls so that he may position probe 10 selectively in alignment with each ball. It is understood that the stoboscopic generator 15 and light 16 may be parts of a commercially available, externally triggerable, stoboscopic light source. Alternatively, instead of using signals from detector 5, a separate adjustable low-frequency signal generator 5' may be employed as the input to notch filter 12 by moving switch 11' appropriately. The adjustment of the output of signal generator 5' to a desired frequency may be manually accomplished by adjustment 5" in the conventional manner.

In addition, the pulsed output of power amplifier 13 is used, according to the invention, to trigger pulse generator 26 which, when switch 34 is closed, supplies high energy sonic pulsed carrier signals via leads 24, 25 to the ultrasonic driver 3 employed in the invention. Pulse generator 26 produces a suitable high frequency carrier pulse of several cycles duration. Such a pulse may, for example, comprise a 100mHz. frequency pulse of about 10 nanoseconds time duration. It is evident that these high frequency pulses are synchronized by and in effect gated at the ball pass frequency when switch 34 is closed. The pulse signals on leads 24, 25 are used to synchronize ultrasonic driver 3 which operates as will be described in more detail in connection with FIGS. 3, 4, and 5 to develop pulsed strains of the piezoelectric ceramic element constituting its active element. Driver 3 may be mounted or supported with respect to frame 14 or may be hand held. The strain pulse is focused by driver 3 into the non-rotating bearing race under test through the end 10 of its probe. Since the surface of end 10 is positioned against the non-rotating bearing race at a position determined by use of the Strobe lamp 16, each pulse of ultrasonic energy is directed to pass through the ball-lubricant interfaces of the bearing under test. It will be understood that pulse generator 26 may be operated independently if supplied with a conventional internally triggered oscillator, such as a pulsed oscillator whose repetition rate is selected according to the manual setting of control knob 26a.

In the illustrated embodiment of a typical bearing test apparatus, the flexible mount 2, as seen in FIG. 2, is characterized by a base 8 normally mounted on a solid foundation and supplied with upwardly projecting flexible legs 28, 28a rigidly fastened at their lower ends to base 8 and left free at their upper ends to flex as indicated by the respective arrows 31, 31a. The legs 28, 28a serve to support the frame 14 on which the gyroscopic rotor assembly is mounted for test, frame 14 being attached to the upper ends of legs 28, 28a by the respective flexure elements 29, 29a. The cradle or mount 14 may be made in two mirror image clamping parts 30, 30a held together and fastened to flexures 29, 29a by conventional fasteners. The ball pass frequency sensor 5 may be mounted on base 8 by a bracket 27 so that its sensing face is located at 5b in contact with an armature element 5a attached to frame 14. Oscillation or vibration of frame 14 in the sense of arrows 31, 31a produced by forces within the bearing produces corresponding output signals within sensor 5 that are supplied to amplifier 11. For example, the most frequently used output signals would be associated with the ball pass frequency or the cage or the ball set frequency.

A preferred form of the ultrasonic driver 3 is that separately disclosed and claimed in the P. E. Jacobson patent application S. N. 544,979 for an "Ultrasonic Driver Transducer," filed Jan. 29, 1975 and assigned to Sperry Rand Corporation; this ultrasonic transducer is preferably used because the relatively short ultrasonic carrier pulses may be generated with no significant generation in the interior of the driver of undesired reflections or spurious resonance effects that would tend to mask the generation of useful response signals by the bearing unit under test. The novel ultrasonic driver is shown in detail in FIGS. 3 and 4 of the present application, the latter figure providing an isolated view of certain of the elements essential to the generation and propagation of the high frequency sound waves, as will be further explained. Referring particularly to FIG. 3, the electrical leads 24 and 25 coupled to the pulse generator 26 of FIG. 1 enter the protective casing 52, the interior of which supports a coaxial transmission line fitting 53 having a flanged portion 54. Flanged portion 54 is fixedly supported by the threaded ring clamp 55 so that the inner conductor 61 of coaxial line fitting 53 is urged against an end of spring 62. The threaded clamp 55 is held by threads 56 on the doubly threaded ring element 57 which is, in turn, supported in the main body 60 of the driver on internal threads 59. The protective casing 52 covering transmission line fitting 53 is sealed at 58 to body 60 by any convenient sealant or is otherwise fastened thereto.

The main body 60 of the instrument is provided with a generally hollow axial cavity for accommodating elements for performing several functions. The first feature of the cavity region lies in the support of a solid piezoelectric circular cylinder 76. Cylinder 76 is equipped on its opposite flat sides with thin electrically-conducting film electrodes 75 and 77 which may, for example, be made of silver or other electrically conducting materials by conventional fabrication methods. The cylinder 76 is preferably constructed of a lead zirconate-titanate compound well known to have suitable piezoelectric characteristics and generally known on the market by the designator PZT-4. The pulsed signals from electrical pulse generator 26 are applied via leads 24 and 25 through spring conductor 62 and via lead 70 through seal 78 to the respective opposed thin film electrodes 75 and 77.

The piezoelectric material of cylinder 76 is polarized in such a manner as to produce bidirectional axial strains when an alternating potential is applied to electrodes 75, 77. This axial strain produces plane waves flowing into the end of lens 79 contacting electrode 77. The opposite face of cylinder 76 at electrode 75 is clamped by ring member 57 surrounding the cavity volume so as to provide support of the piezoelectric cylinder 76 without diminishing the intensity of the developed strain.

For purposes of focusing the ultrasound waves generated by piezoelectric cylinder 76, a converging plano-concave lens 79 of aluminum and of generally cylindrical shape is provided with a concave front surface 80. Generally cylindrical in shape like cylinder 76, lens 79 is held in place against the thin film electrode 77 at the radiating surface of cylinder 76, being compressed against annular seal 81. The planar surface of aluminum lens 79 is preferably lapped and cemented to film electrode 77, the two flat surfaces of cylinder 76 also having been smoothly lapped prior to the installation of electrodes 75 and 77. The cement used to fasten lens 79 to film 77 may be a commercially available epoxy resin providing a close acoustical impedance match between the ceramic of cylinder 76 and the aluminum material of lens 79, also closely matching the thermal expansion coefficients of these materials.

Materials of the type preferred for forming the piezoelectric cylinder 76 may tend to generate objectionable abnormal mode surface waves propagating normal to the desired direction of acoustical energy propagation. For this reason, a fluid interface coupling cavity region 82 is provided, because the undesired shear waves tend to be attenuated within the liquid and are therefore not propagated. Furthermore, it is desired to concentrate the energy emanating from concave surface 80 to flow through a relatively small cross section area because such an arrangement facilitates putting maximum energy into a small surface area of a device under test. The fluid interface volume 82, 94 is supplied primarily by the generally conical cavity portion 82 and the continuing cylindrical cavity portion 94 and these volumes are filled with glycerin. With the cavity portions 82 and 94 and the space in front of concave surface 80 filled with glycerin, the acoustical path is focused in a converging manner towards spherical input surface 103 in the general manner shown clearly in FIG. 4. Here, the extremum rays 110, 111 of the ultrasound beam emanating from the piezoelectric cylinder 76 are seen to be focused by plano-concave lens 79 toward the convex input surface 103 as they pass through coupling cavities 82, 94. The rays 110, 111 impinge upon the front convex surface 103 of coupler rod 105, which rod may be construced of steel. Because of the curved convex face 103, rays 110 and 111 are collimated and run as parallel rays with parallel wave fronts through steel coupler rod 105. An object placed against face 106 of coupler rod 105 is thus desirably subjected to pulses of ultrasonic energy. A drop of glycerin may be placed on coupler rod face 106 to furnish a close impedence match between steel coupler rod 105 and a surface of the ball bearing or other element to be excited by ultrasonic energy for test purposes. When in use, rod 105 is normally depressed by contact forces by the object to be excited until its active output face 106 is nearly flush with face 109 of flange 107. When rod 105 is so positioned, the surfaces 80 and 103 of the associated lenses are focused properly to assure maximum energy transfer through the system and out through rod 105. This arrangement assures the existence of a fixed and suitable contact force of the face 106 against the object under test, with no degradation of performance and no introduction of operator error.

The high quality steel coupler rod 105 is supported within a glycerin-filled extension of cavity 94 enclosed by casing 101 affixed to the end of body 60 by fasteners such as screw 100. Concentrically mounted in casing 101 on flange 107 is a flexible bellows 102 enclosing steel coupler rod 105 and sealed thereto at the circumferential face 104 adjacent its front convex surface 103. The volume within casing 101 and exterior of coupler rod 105 is also filled with glycerin, that within bellows 102 simply enclosing air. Coupler rod 105 is supported by flange 107 in a hollow metal bushing 108 for free axial translation therein. Bushing 108 is affixed rigidly to flange 107 by a suitable adhesive, for example. Thus, the acoustic energy is focused by the aluminum lens 79 through the glycerin within cavities 82 and 94 in an acoustically matched path and on through the fluid medium into the convex surface 103 of coupler rod 105, wherein it propagates in the form of parallel rays through the planar outer surface 106 into the object to be tested. The body 60 of the instrument is adapted to be supported with respect to the bearing element to be excited by a suitable rigid support such as illustrated generally at 112, or may be hand held.

Elements branching radially from cavity 82 are employed in introducing glycerin into the internal cavities of the ultrasonic driver and include a gas purging device and a sealing tube. The former consists of a casing 86 sealed at 87 within body 60 and containing a coaxially mounted bellows 88. Bellows 88 is affixed at its upper end to the circumferential face of a flanged member 95; member 95 is supplied with a bore 85 communicating between volume 82 and the interior of bellows 88. The opposed end of bellows 88 is sealed to the circumferential face at the flanged end 90 of plunger or piston 89. So that piston 89 may be translated along its axis, a threaded rod 91 is held for manual rotation in the end 92 of casing 86, the threaded part of rod 91 cooperating with plunger 89. Rotation of knob 93 on threaded rod 91 moves plunger 89 toward or away from cavity 82, respectively urging liquid into it, or withdrawing liquid.

To prevent permanent entrapment of gas within the interior of the device, the bellows 88 is gradually compressed while the unit is actually operating with electrical driving signals on leads 24, 25 until all entrained gas is expelled through the open pinch-off tube 83 located in bore 84 diametrically opposite the purging apparatus. When all entrained gases are expelled, tube 83 is sealed, the pinch-off tube 83 and its method of use being of the kinds conventionally used in vacuum tube and other arts. The fact that the body 60 of the driver and casing 101 are constructed of a transparent material such as a methacrylate permits the operator to observe visually the presence of entrapped gas and to tilt or otherwise manipulate the device so that such gas exits through the open pinch-off tube 83, after which tube 83 is sealed.

As discussed in the foregoing, each bearing ball represents a mass supported by the inner and outer bearing raceway contact surfaces and by the corresponding elastohydrodynamic lubricant films, if present. Since these several mechanical elements contribute to the resonance frequency of a system including both of the races, each bearing ball, and each lubricant interface, a resonant condition of the combined system may readily be established. While this resonant frequency is clearly related to characteristics of the metal parts of the bearing, it is also found to be related in a fundamental way to the presence and nature of the lubricating film of the bearing. In addition, the character of the bearing surfaces themselves provides similar measurable influence.

As seen in FIG. 1, components of vibration due to any sources in the bearing 7 under test may be picked up by a conventional sensitive vibration pick-up 4, such as a high resonance frequency accelerometer device, mounted on frame 2 and having a surface 9 in contact with the outer race of bearing 7. The output of pick-off 4 is heterodyned with a suitable reference frequency in a conventional mixer 18 which may include conventional adjustable band pas output filters to produce an output signal preferably in the audio range. A second input to mixer 18 is the output of oscillator 17 which has an adjustment 17a permitting its operating frequency to be varied over a relatively wide range. The beating frequency from oscillator 17 is adjusted to beat with the ball-race-lubricant system resonance frequency in one type of operation of the invention, so that the resultant beat frequency is related to the lubricant film characteristic or condition. The beating frequency after amplification by power amplifier 19, if desired, may be selectively switched by operating particular switches of array 36 to a conventional frequency analyzer or, as shown in the figure, may be selectively supplied to other conventional analysis instruments, such as dB. meter 20, loud speaker 21, recorder 22, or cathode ray oscilloscope 23, the latter normally being synchronized by the output of pick-off 5 when switch 35 is closed. In actual use, the oscillator frequency may be adjusted to provide a null output or a fixed audible output frequency when referenced to a stationary bearing and a zero elastohydrodynamic lubricant film condition. As the bearing is accelerated in speed, the beat frequency produced is representative of the instantaneous lubricant film condition. Various modifications of this procedure provide data relatable to bearing alignment, preload, raceway wetability, lubricant film integrity, temperature sensitivity, and geometric imperfections of balls and raceways.

The lubricant film stiffness is considerably greater than the stiffness developed by the ball-raceway contact surfaces; the latter may be referred to as the contact spring. Since the effective contact spring and the lubricant spring are in mechanical series, the two contact springs are in parallel and the two lubricant springs are also in parallel for each ball; as the lubricant film thickness diminishes, the system ball-to-race stiffness ratio approaches the same value as found in a stationary bearing. The ball-race system resonance frequency decreases as the bearing speed increases from zero due to the formation of the lubricant film. This resonance value falls off continuously with speed, asymptotically approaching a steady state value less than the static resonance. If the resonance frequency should remain fixed or begin to increase as speed continues to be increased, it would be indicative of a degraded lubricant film condition due, for example, either to axial preload increases or to operating temperature increases.

FIGS. 6 and 7 are graphs showing the stiffness contributed by the elastohydrodynamic lubricant film alone related to axial loads and operating temperatures for the representative lubricant and 34BX4 (R4 size) and 304H bearings, respectively. These bearings were chosen to illustrate results for bearings of substantially different sizes. The graphs do not include any influence of contact compliance, but show interesting characteristics of the stiffness contributed by the lubricant film. The film stiffness (and the associated ball-race-film resonance frequency) is almost independent of axial load for operating temperatures around room temperature for the particular lubricant and bearings represented. In general, all elastohydrodynamic lubricant films become stiffer as temperature increases and as axial load increases in the region of lubricant film breakdown, the latter occurring at the highest temperatures. Thus, the lubricant film thickness between the individual bearing balls and the bearing race is determined by measuring the resonant frequency of the total spring-mass system in an active bearing, as compared to a static bearing. This resonant frequency is related to the mass of the bearing ball in conjunction with the two fluid springs at the bearing interfaces. The resonance of the total spring-mass system is in the ultrasonic range and is not to be confused with the resonance frequency of the bearing balls alone, which is a discrete self-resonance and is not related to the elastohydrodynamic lubricant film thickness, but is of much higher frequency.

The bearing ball-race-lubricant system may be excited at system resonance by way of example as illustrated in FIG. 8, which shows a fragment of one type of gyroscopic rotor assembly in which the rotor 6 is supported on a rotatable shaft 120 having an end extension 121 fixed in the rotatable race 7b of ball bearing 7. Parts of an electrical drive motor (not shown) normally occupy a portion of the hollow interior region 123 of rotor 7. The assembly includes a casing 122 with an extension 124 for supporting the stationary race 7a of bearing 7, the bearing balls 7c lying between races 7a and 7b in the usual manner.

The face 10 of driver 3 may be placed in direct contact with stationary race 7a so that the ultrasonic pulses from driver 3 enter race 7a and couple to bearing balls 7c and to race 7b. When the proper carrier frequency is selected, the ultrasonic energy excites the system made up by the balls and the associated lubricated raceways. The amplitude of resonant oscillation or ringing is detected by placing the vibration pick-up probe 4 so that face 9 also contacts part of stationary race 7a, for example.

In the arrangement of FIG. 8, a bearing protection or access cover (not shown), has conveniently been removed from the cooperating access aperture 125, as may also conveniently be provided in other rotating instruments it is desired to test, such as in precision motors or the like. In instances in which access to the bearing by driver 3 and pick-off 4 is difficult, the driving signals and the consequent system resonating signals may be transmitted with respect to the bearing through a bearing support element such as part 124 of casing 122. In configurations in which the outer bearing race is stationary and the inner race supports the rotating equipment, probe elements 3 and 4 may again readily be applied either directly to the stationary race or to a stationary element in intimate contact therewith. It will be understood that driver transducer 3 may be positioned for various purposes in a variety of positions simply by hand holding at the desired position or by using only ordinary mechanical clamping arrangements fixed to frame 14, and that the pick-up probe 4 may also be hand held or clamped in fixed relation with respect to frame 14 in a similar manner in any of a variety of positions.

The test apparatus benefits particularly by use of a driver 3 such as that described in the aforementioned patent application in which relatively short ultrasonic carrier pulses may be generated with no generation internal of the driver device of undesired resonance effects or multiple reflections that would subject the race being excited to delayed signals that would undesirably mask the true ball-race-lubricant system signals. It is also seen that the novel test apparatus is of highly flexible nature, facilitating a variety of types of measurements of the characteristics of bearing-rotor assemblies. The meters 20 through 23 may be used cooperatively, for example, for bearing raceway searches to reveal race geometry irregularities and the existance of areas of the raceway surface where lubricant is absent, as well as the degree to which the elastohydrodynamic lubricant film has or has not been established. These tests are accomplished with rotor 6 revolving by searching the stationary race with the pick-off probe 4 and observing or listening to the amplitude of the audio output on dB meter 20, the audio tone on speaker 21 or recorder 22, or by observing the resonance frequency itself on a spectrum analyzer which may be incorporated in oscilloscope 23. Such observations may also be made upon oscilloscope 23 when synchronized, by the closing of switch 35, to the repetition rate of the ball pass output from pick-off 5. Race misalignment may be recognized for a given position of pick-up probe 4 by observation on such metering instruments of an audio modulation of the bearing system resonance signal or a frequency modulation or warble of the audio tone or of the bearing-ball-lubricant resonant frequency, the frequency deviation amplitude supplying a measure of the magnitude of the effect to be examined. The integrity of the elastohydrodynamic lubrication film with speed, load, temperature, or other parameter may be determined, the effects of lubricant additives may be determined, or the influence of loss of volatile components of the lubricant may be studied by changing a design parameter of the bearing or changing the lubricant quality or guantity, then measuring the change in the mean frequency or oscillation of the bearing-ball-lubricant system. Meters 20, 22, and 23 are normally employed for the latter purpose. Bearing brinelling of a race is located by exploring the stationary race with probe 4 for disturbances of the lubricating film thickness and observing modulations in the bearing-ball-lubricant resonance frequency or the audio tone.

Accordingly, it is seen that the novel test apparatus and method provides direct measurements of significant characteristics of ball bearings and their lubricants in an environment closely or exactly resembling that in which they are normally used, overcoming the defects of the prior art concepts. The invention depends ideally upon internal system resonance characteristics of the bearing ball-race-lubricant combination in a fundamental manner that avoids spurious effects generally found in prior art concepts which cause them to yield inaccurate measures of the reliability and life time expectancy of ball bearing systems operating in actual instruments assemblies.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects. 

I claim:
 1. Apparatus for the evaluation of characteristics of a ball bearing having first and second bearing race means, said bearing race means being spaced apart by cooperating plural bearing ball means for rotation of said first bearing race means with respect to said second bearing race means about a common axis, said ball bearing further having a spring-mass mechanically resonant system including said first and second bearing race means, at least one of said cooperating bearing ball means, and the lubricant film lying therebetween, said apparatus comprising: motive means for rotating said first bearing race means about said common axis with respect to said second bearing race means, signal generator means, pulsed ultrasonic driver means responsive to said signal generator means for exciting resonance oscillations in said spring-mass mechanically resonant system, and first signal pick-off means for providing a measure of a selected characteristic of said resonance oscillations.
 2. Apparatus as described in claim 1 wherein said pulsed ultrasonic driver means is acoustically coupled to said second bearing race means.
 3. Apparatus as described in claim 2 wherein said signal generator means includes adjustable frequency control means.
 4. Apparatus as described in claim 2 wherein said signal generator means comprises second signal pick-off means responsive to oscillatory translation of said second bearing race means substantially normal to said common axis.
 5. Apparatus as described in claim 2 additionally including adjustable notch filter means in series connected relation with said signal generator means and said pulsed ultrasonic driver means.
 6. Apparatus as described in claim 2 additionally including signal frequency mixer means responsive to said first pick-off means and to adjustable frequency oscillator means.
 7. Apparatus as described in claim 6 additionally including signal display means responsive to said mixer means.
 8. Apparatus as described in claim 7 wherein said signal display means includes signal amplitude display means.
 9. Apparatus as described in claim 7 wherein said display means includes cathode ray oscilloscope display means synchronized in response to said signal generator means.
 10. Apparatus as described in claim 5 including stroboscopic light means responsive to said notch filter means for synchronized illumination of at least a position of said ball bearing with respect to the excitation of said pulsed ultrasonic driver means.
 11. Apparatus as described in claim 1 wherein said first signal pick-off means is acoustically coupled to said second bearing race means.
 12. Apparatus as described in claim 1 wherein said motive means comprises motor rotor means supported for rotation directly within said ball bearing as an element of a production sub-assembly.
 13. Apparatus as described in claim 4 further including flexure means for supporting said second bearing race means with respect to base means for oscillatory translation of said second bearing race means substantially normal to said common axis.
 14. Apparatus for the evaluation of characteristics of a ball bearing having first and second bearing race means, said bearing race means being spaced apart by cooperating plural bearing ball means for rotation of said first bearing race means with respect to said second bearing race means about a common axis, said ball bearing further having a spring-mass mechanically resonant system including said first and second bearing race means, at least one of said cooperating bearing ball means, and the lubricant film lying therebetween, said apparatus comprising:motive means for rotating said first bearing race means about said common axis with respect to said second bearing race means, flexure means for supporting said second bearing race means for oscillatory translation substantially normal to said common axis, signal generator means, pulsed ultrasonic driver means responsive to said signal generator means for exciting resonance oscillations in said spring-mass mechanically resonant system, and first pick-off means for providing a measure of a predetermined characteristic of said resonance oscillations.
 15. Apparatus as described in claim 14 wherein said signal generator means includes adjustable frequency control means.
 16. Apparatus as described in claim 14 wherein said signal generator means comprises second signal pick-off means responsive to said oscillatory translation of said second bearing race means.
 17. Apparatus as described in claim 16 further including frame means for mounting said second bearing race means on said flexure means for oscillation with respect to base means.
 18. Apparatus as described in claim 17 wherein said pulsed ultrasonic driver means and said first signal pick-off means are supported by said frame means.
 19. Apparatus as described in claim 18 wherein said second signal pick-off means is mounted on said base means.
 20. Apparatus as described in claim 16 additionally including signal frequency mixer means responsive to said first signal pick-off means and to adjustable-frequency oscillator means.
 21. Apparatus as described in claim 20 additionally including signal display means responsive to said mixer means for displaying said predetermined characteristic.
 22. Apparatus as described in claim 16 including stroboscopic light means responsive to the said second signal pick-off means for synchronized illumination of at least a portion of said ball bearing.
 23. The method of testing a rotary bearing having first and second bearing race means, said bearing race means being spaced apart by a cooperating plurality of bearing ball means for rotation of said first bearing race means with respect to said second bearing race means about a common axis, said rotary bearing further having a spring-mass mechanically resonant system including said first and second bearing race means, at least one of said cooperating bearing ball means, and the lubricant film lying therebetween, including the steps of:rotating said first bearing race means with respect to said second bearing race means, determining an instantaneous angular location of one of said bearing ball means with respect to said second bearing race means, applying a pulse of ultrasonic energy to said second bearing race means at said instantaneous angular location in the presence of said one bearing ball means for exciting resonant oscillations in said spring-mass mechanically resonant system, and detecting said resonance oscillations to determine an operating characteristic of said rotary bearing.
 24. The method of testing a rotary bearing described in claim 23 wherein said step of rotating said first bearing means includes the sub-step of resiliently supporting said second bearing means for oscillatory translation substantially normal to said common axis.
 25. The method of testing a rotary bearing described in claim 24 wherein said step of determining an instantaneous location of one of said bearing ball means includes pulsed illumination of said bearing ball means.
 26. The method of testing a rotary bearing described in claim 25 wherein said pulsed illumination is applied synchronously with respect to said oscillatory translation.
 27. The method of testing a rotary bearing described in claim 25 wherein said pulse of ultrasonic energy is applied synchronously with said oscillatory translation.
 28. The method of testing a rotary bearing described in claim 27 wherein said step of detecting said resonance oscillations includes the sub-steps of:converting said resonance oscillations to audible frequency oscillations, and displaying said audible frequency oscillations with respect to a selected characteristic thereof.
 29. The method of testing a rotary bearing supporting a rotor member for rotation about a spin axis, said rotary bearing comprising a stationary bearing race resiliently supported for motion in a direction normal to said spin axis, a rotatable bearing race on said rotor member, and a plurality of spaced ball elements between said bearing races, said rotary bearing being under normal assembled preload for said rotor member, said method consisting in:rotating said rotor member about said spin axis at an arbitrary rotation frequency, determining the angular position with respect to said spin axis of a selected ball element relative to said stationary bearing race, applying a pulse of ultrasonic carrier energy to said stationary bearing race adjacent said selected ball element when at said angular position whereby said selected ball element in combination with said bearing races oscillates in response to said ultrasonic energy in a pattern characteristic of said combination, and detecting a characteristic of said pattern for determining an operating condition of said rotary bearing.
 30. The method of testing a rotary bearing described in claim 29 wherein the carrier frequency of said ultrasonic carrier energy is much higher than said arbitrary rotation frequency.
 31. The method of testing a rotary bearing described in claim 29 wherein pulses of ultrasonic carrier energy are applied to said stationary bearing race at a pulse repetition rate which is a linear function of said arbitrary rotation frequency.
 32. The method of testing a rotary bearing described in claim 29 wherein pulses of ultrasonic carrier energy are applied to said stationary bearing race at said angular position whereby said rotary bearing is induced to oscillate each time a ball element passes said angular position whereby, upon detection of the induced oscillations, an operating condition of said bearing may be determined. 